Spinach CSP41, an mRNA-binding protein and ribonuclease, is
homologous to nucleotide-sugar epimerases and hydroxysteroid
Michael E. Baker
William N. Grundy
Charles P. Elkan
Biochemical and Biophysical Research Communications,
Spinach CSP41 is part of a protein complex that binds to the 3'
untranslated region (UTR) of petD precursor-mRNA, a chloroplast gene
encoding subunit IV of the cytochrome b 6/f complex. CSP41 cleaves
the 3'-UTR of petD mRNA within the stem-loop structure, suggesting a
key role in the control of chloroplast mRNA stability. We discovered
that CSP41 is homologous to nucleotide-sugar epimerases and
hydroxysteroid dehydrogenases while seeking distant homologs of these
enzymes with a hidden Markov model-based search of Genpept. This
analysis identified Synechocystis ORF, Accession 1652543 as a homolog.
Subsequent analyses show that spinach CSP41 and Arabidopsis thaliana
2765081 are homologous to the Synechocystis ORF. Information from the
solved 3D structures of epimerases and dehydrogenases and our motif
analysis of these enzymes is used to predict domains on CSP41 that are
important in binding and metabolism of mRNA. Cyanobacteria are among
the earliest life forms, indicating that the divergence from a common
ancestor of nucleotide-sugar epimerases and an mRNA binding protein
with ribonuclease activity was ancient.